The present invention relates to an objective lens used for an optical disc drive, which is capable of recording data to and/or reproducing data from a plurality of types of optical discs having different recording densities and having different thicknesses of cover layers.
There are various types of optical discs on which digital information is recorded at various densities, respectively. For example, a DVD (digital versatile disc) has a recording density higher that that of a CD (compact disc) or a CD-R (CD Recordable) and has a cover layer thinner than that of the CD or CD-R.
When recording/reproducing operation for the DVD having a higher recording density is performed, a smaller beam spot diameter is required on a data recording layer of the DVD relative to a beam spot used for the CD having a lower recording density. For this reason, the optical disc drive is configured such that an NA (numerical aperture) is changed to a higher value to obtain a smaller beam spot diameter when the DVD is used and that the NA is changed to a lower value to obtain a larger beam spot diameter when the CD or CD-R is used.
A condition of a spherical aberration in an optical system of the optical disc drive changes depending on a thickness of the cover layer of the optical disc being used. Therefore, it is required to correct the spherical aberration caused when the optical disc is changed to another one having different thickness of the cover layer.
The diameter of the beam spot decreases as a wavelength of a laser beam incident on the objective lens decreases. Therefore, in general, a laser beam having a wavelength ranging from 635 nm to 660 nm is used for the DVD, and a laser beam having a wavelength ranging from 780 nm to 830 nm is used for the CD. By using such wavelengths, a relatively small beam spot is obtained when the DVD is used, and a relatively large beam spot is obtained when the CD is used.
In general, the optical disc drive is provided with a light source configured to emit laser beams having different wavelengths in order to support different types of optical discs.
Japanese Patent Provisional Publication No. 2001-243651 discloses an objective lens configured to suitably converge incident laser beams onto data recording layers of a plurality of types of optical discs having different thicknesses of cover layers, respectively. On one of lens surfaces of the objective lens disclosed in this publication, a diffracting structure having a plurality of ring-shaped minute steps is formed. In an optical system disclosed in the publication, each of two collimated laser beams having different wavelengths is incident on the objective lens.
In order to form beam spots more suitable for recording data to and/or reproducing data from the plurality of types of the optical discs, the objective lens is required to be corrected for a coma as well as the spherical aberration. The coma is caused when off-axis light is incident on the objective lens (i.e., when a beam is incident on the objective lens obliquely with respect to an optical axis of the objective lens).
However, the objective lens disclosed in the publication 2001-243651 can not suitably correct the comas for all of the plurality of types of optical discs. More specifically, the objective lens disclosed in the publication is configured to adjust balance of the coma considering the intended use of an optical disc drive including the objective lens.
Japanese Patent Provisional Publication No. 2003-156682 discloses an objective lens configured to support the plurality of types of the optical discs and to reduce the coma caused when off-axis light is incident thereon. Each of lens surfaces of this objective lens has an inner region and an outer region located outside the inner region.
The inner region is a region for attaining an NA for a second optical disc having a lower recording density. The outer region is a region for attaining an NA for a first optical disc having a recording density higher than that of the second optical disc. The inner region and the outer region of each of the lens surfaces of the objective lens have different shapes.
In the publication, it is described that the coma caused when the off-axis light is incident on the objective lens is reduced by the above mentioned structure. However, in the publication No. 2003-156682, no explanation is made on how to configure the shapes of the inner region and the outer region of each lens surface to effectively reduce the coma.